DESCRIPTION: The past four years of NIH/NIDA support has enabled a highly successful translational research study of acute hepatitis C virus (HCV) infection in young injection drug users (IDU). This application proposes to build on and expand our scientific aims. HCV remains a problematic human pathogen and there is good evidence that this epidemic, after years of decreasing, it is expanding. HCV incidence has been high in many urban areas, for instance in San Francisco, in our UFO Study cohort, HCV incidence among young adult IDU has been stable but unacceptably high at 23% for over 10 years. Numerous outbreaks as well as surveillance has shown increases in injection and HCV infections in multiple suburban and rural areas, so much so that in February of this year, U.S. Department of Health and Human Services Office of the Assistant Secretary for Health, Office of HIV/AIDS and Infectious Disease Policy held a technical consultation on Hepatitis C Virus Infection in Young Persons who Inject Drugs bringing together experts to address this emerging epidemic. Increases are associated with burgeoning prescription opioid epidemic with high rates of transition to IDU, as well as high risk injecting practices, and limited comprehensive prevention strategies. This population is also at risk for HIV, and although rates are much lower than for HCV (<0.8%), but this resurgence in risk and HCV rates raises concerns that gains in HIV prevention could be reversed. In this competitive renewal of the UFO Study, a NIDA funded observational cohort focused on HIV and HCV in young IDU for the past 10 years, we propose to continue and expand our comprehensive prospective investigation of risk and disease outcomes and HCV immunology. The UFO Study is one of the most successful studies of incident and acute HCV globally. The high incidence of infection and our successful systematic data collection provides a unique opportunity to study and advance multiple scientific approaches to HCV and HIV prevention. We successfully pioneered the systematic use of anti-HCV and qualitative HCV RNA testing to identify acute incident HCV, during the pre- seroconversion period, when viral RNA levels are high, but anti-HCV is not detectable. In this application we propose to build on our successful implementation of novel testing methodologies to study how rapid HCV tests may impact risk behavior, HCV incidence, and HIV testing (Aim 1). Preliminary data suggests that this new technology is associated with reductions in injecting risk and increased HIV testing. For aim 2, we will further develop our successful and growing cohort of serodiscordant injecting partners to allow for in-depth examination of HCV transmission; specifically to assess how viremia drives infection. Our 3rd and 4th aims with collaborators at Harvard and Johns Hopkins Universities, we propose two lines of immunological studies of protective immunity: (1) to test the hypothesis that inherent differences in the T cell response between women and men contribute to the higher likelihood of spontaneous clearance of HCV infection in female subjects, and (2) to study mechanisms underlying reclearance of HCV after reinfection. The UFO Study is uniquely poised to conduct and collaborate on these immunological studies because of our well characterized cohort, the ability to systematically sample high risk young IDU, to frequently and cost-effectively sample for HCV RNA and anti- HCV, and the high rate of acute infections detected, all of which are necessary to detect spontaneous clearance, reinfection, and reclearance. As in the past, we will place an emphasis on the role that biological sex plays with divergent infection outcomes, and for the first time will collect data on hormones and serum ferritin that we will use to assess potential mediating or direct effects the divergent outcomes to infection seen by sex. All of these aims will inform critical aspects of HCV and HIV prevention, including testing and counseling strategies and strategies to reduce transmission in injecting partnerships. The immunology studies will provide information that will inform HCV vaccine development and potentially other viral infections including HIV. With ongoing and expanding transmission of HCV, there is a critical need to more fully inform and more fully develop a prevention toolbox to reduce blood borne infections and associated burden of disease in people who inject drugs.